The present invention relates to a vehicular automatic transmission for automatically shifting gears by changing power transmission paths through engagement and disengagement of gear shift means (e.g., hydraulically operated clutches).
Automatic transmissions are arranged to shift gears automatically depending on running conditions of a motor vehicle to achieve desired vehicle running characteristics. It is customary to provide a gearshift map composed of upshifting and downshifting curves for each gear position, the curves being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the gears according to the gearshift map dependent on the running conditions as indicated on the gearshift map. One example of such gear shifting control is disclosed in Japanese Laid-Open Patent Publication No. 61-189354, for example.
One type of automatic transmission includes a power transmission means comprising a plurality of power transmission paths (e.g., a plurality of gear trains), a plurality of gearshift means (e.g., a plurality of hydraulically operated clutches) for selecting the power transmission paths, and a control means (e.g., a hydraulic pressure control valve) for controlling operation of the gearshift means. When a running condition of a motor vehicle, as indicated on a gearshift map, moves across an upshifting or downshifting curve, a gearshift command is produced to effect an upshift or downshift, and a solenoid valve is operated based on the gearshift command to control operation of the hydraulic pressure control valve to engage one of the hydraulically operated clutches. The power transmission path through a certain gear train associated with the engaged clutch is now selected to effect a gearshift.
The speed reduction ratio (gear ratio) of a previous gear position (i.e., a gear position provided by the power transmission path (gear train) selected until a gearshift command is issued) is different from the speed reduction ratio of a next gear position (i.e., a gear position provided by the power transmission path selected by the gearshift command). Therefore, when such a gearshift is effected, it is necessary that the automatic transmission be controlled so as not to produce a gearshift shock and a gearshift delay.
It is also proposed to connect an accumulator to the hydraulically operated clutch to allow the engaging clutch torque for the next gear position to vary gradually for smooth engagement of the next-gear-position clutch, or to release the hydraulic pressure from the previous-gear-position clutch depending on a hydraulic pressure buildup in the next-gear-position clutch, or to control the hydraulic pressure to be supplied to the clutch depending on the engine output power (see Japanese Laid-Open Patent Publication No. 60-211152, for example).
An automotive automatic transmission effects various gearshifts, and the torque required to engage gearshift means differs from gearshift to gearshift. It is difficult to obtain desired gearshift characteristics for all gearshifts through the control using the accumulator, the orifice control valve, or the like, or the control of the clutch hydraulic pressure depending on the engine output power.
There are known gearshifts in a power-on/downshift mode in which the accelerator pedal is depressed and the transmission is shifted down (corresponding to a kickdown). A gearshift in such a power-on/downshift mode is controlled as follows: When a gearshift command is generated, the previous-gear-position clutch (the clutch which has been engaged so far) is disengaged, and when the rotational speeds of the input and output members of the next-gear-position clutch (the clutch which will newly be engaged by the gearshift) are synchronized with each other, the next-gear-position clutch is engaged. According to such a gearshift control process, the gearshift can smoothly be effected since no inertial energy is transferred between the input and output members of the next-gear-position clutch when it is engaged. The clutches referred to above are gearshift means. The brake of a planetary transmission mechanism also corresponds to a gearshift means in an automatic transmission.
If, however, the time at which the input and output rotational speeds of the next-gear-position clutch are synchronized is detected by a timer or based on the relationship between the vehicle speed and the engine rotational speed, then the accuracy of detection is low because of the oil temperature, different characteristics of individual gearshift means, and slippage in the torque converter and fluid coupling. If the speed synchronizing time is detected in error, then the next-gear-position clutch may be engaged too early resulting in a gearshift shock, or may be engaged too late resulting engine racing or making the driver embarrassed.
In view of the above drawbacks, it has been proposed to detect the ratio of the rotational speed of an input rotatable member to the rotational speed of an output rotatable member of a hydraulically operated clutch (the ratio=output rotational speed/input rotational speed), and determine when the input and output rotational speeds are synchronized by detecting that the input and output rotational speed ratio becomes substantially 1.0 (Japanese Patent Application No. 63-50337). According to this proposal, when a gearshift is to be effected in the power-on/downshift mode, for example, a hydraulic pressure supplied to the next-gear-position clutch is kept at a level slightly lower than a hydraulic pressure to start engaging the clutch after the gearshift is started until the synchronization of the input and output rotational speeds is detected. After the synchronization of the input and output rotational speeds is detected, the hydraulic pressure supplied to the next is increased to a predetermined clutch engaging pressure.
The above gearshift control for the automatic transmission can considerably accurately detect the time when the input and output rotational speeds are synchronized since the input and output rotational speed ratio of the next-gear-position clutch is directly detected by detecting the rotational speeds of the input and output rotatable members thereof. If the speed synchronizing time is detected slightly in error or the timing at which the next-gear-position clutch starts to be engaged is slightly shifted, since the input and output rotational speed ratio varies sharply, such an error causes a gearshift shock or engine racing even if the error is small.
In view of the above problem, it may be proposed not to increase the hydraulic pressure directly to the clutch engaging pressure when the synchronized speeds are detected, but to set the hydraulic pressure to a level commensurate with the torque transmitted from the engine. With this arrangement, since the clutch is gradually engaged even if the clutch hydraulic pressure is increased slightly early, any gearshift shock produced is small and no engine racing takes place.
In the power-on/downshift mode, at least during an initial stage of a gearshift, the previous-gear-position clutch is disengaged and the next-gear-position clutch is not engaged and does not transmit any torque. Therefore, the engine is racing, and the engine torque detected at this time is of a low value. Accordingly, the above problem cannot be solved by setting the hydraulic pressure to a level commensurate with such an engine torque.
There are also known gearshifts in a power-off/upshift mode in which the accelerator pedal is released and the transmission is shifted up. A gearshift in such a power-off/upshift mode is controlled as follows: When a gearshift command is generated, the previous-gear-position clutch is disengaged, and when the rotational speeds of the input and output members of the next-gear-position clutch are synchronized with each other, the next-gear-position clutch is engaged. According to such a gearshift control process, the gearshift can smoothly be effected. However, because any clutches are not engaged after the gearshift is started until the next-gear-position clutch is engaged, if the accelerator pedal is depressed during such an interval of time, the throttle opening is increased and the engine rotational speed is also increased quickly, resulting in engine racing or a gearshift shock caused by such engine racing.
To avoid the above drawback, if the accelerator pedal is depressed after the gearshift is started, the hydraulic pressure supplied to the next-gear-position clutch may be increased to apply to the clutch an engaging torque that is commensurate with the torque transmitted from the engine, so that engine racing is suppressed and a gearshift shock is prevented. According to this control process, however, immediately before the accelerator pedal is depressed, any of the clutches are not engaged, and the engine torque detected at this time is a low value necessary to take up the inertia of rotating parts. Any hydraulic pressure established so as to be commensurate with such an engine torque is not effective to prevent the aforesaid problems such as engine racing.
If the hydraulic pressure is released slowly from the previous-gear-position clutch so that the disengagement of this clutch is delayed, the engagement of the next-gear-position clutch is also delay as much, with the result that a gearshift time lag may be generated. In the power-on/downshift and power-off/upshift modes, since any gearshift is effected by the driver operating on the accelerator pedal (i.e,. depressing or releasing the accelerator pedal), a gearshift is based on the intention of the driver. Accordingly, the above gearshift time lag is easily sensed by the driver, who then feels bad about the gear shifting operation of the transmission.